Organic semiconducting (OSC) materials are receiving growing interest mostly due to their rapid development in the recent years and the lucrative commercial prospects of organic electronics.
One particular area of importance is organic photovoltaics (OPV). Conjugated polymers have found use in OPVs as they allow devices to be manufactured by solution-processing techniques such as spin casting, dip coating or ink jet printing. Solution processing can be carried out cheaper and on a larger scale compared to the evaporative techniques used to make inorganic thin film devices. Currently, polymer based photovoltaic devices are achieving efficiencies above 8%.
In order to obtain ideal solution-processible OSC molecules two basic features are essential, firstly a rigid π-conjugated core or backbone, and secondly suitable functionality of the aromatic cores in the OSC backbone. The former extends π-π overlaps, defines the primary energy levels of the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO), enables both charge injection and transport, and facilitates optical absorption. The latter further fine-tunes the energy levels and enables solubility and hence processability of the materials as well as π-π interactions of the molecular backbones in the solid state.
A high degree of planarity reduces the energetic disorder of OSC backbones and accordingly enhances charge carrier mobilities. In prior art most of the polymeric OSCs with high charge carries mobilities are generally composed of fused ring aromatic systems, and are semicrystalline in their solid states. Such polymers are for example indacenodithiophene-benzothiadiazole copolymers, for which it was reported by Zhang et al., J. Am. Chem. Soc., 2010, 132(33), 11437 that a hole mobility of 1 cm2/V s was achieved.
Nevertheless, the structures of solubilising groups (e.g., the length, the regio-regularity, the spacial orientation of the alkyl chains etc.), have direct effects on the solubility and hence the processibility of the OSC, on the planarity of the polymer backbone, on the inter-chain π-π interactions and on the HOMO-LUMO levels/bandgaps. For many applications, like e.g. OPV devices, optimisation of the electronic properties of the conjugated backbones by fine-tuning the solubilising functional groups can result in dramatic effects on the efficiencies.
The conventional method of introducing solubilising groups into cyclopentadiarene units like indacenodithiophene (Zhang et al., J. Am. Chem. Soc., 2010, 132(33), 11437), is to alkylate the sp3 carbon atoms of the cyclopentadienes contained in these fused ring structures. Due to the tetrahedral configuration of this carbon, the substituents have to take the orientation within a plane that is normal to the aromatic plane of the conjugated backbone, as shown by X-ray single crystal analysis by Hughes et al., Org. Biomol. Chem., 2003, 1, 3069. These out-of-plane alkyl chains increase the inter-planar separation of the π-π backbones, reducing the degree of inter-molecular π-π interactions. However, from a synthetic point of view, multiple alkylation like for example tetraalkylation of the indacenodithiophene leads to difficulties of purification of the expected products due to the very similar polarities of the product and the incompletely alkylated impurities. Partially alkylated fluorene units are prone to form keto defects within the polymer (Scherf et al, Adv. Mater., 2002, 14, 374).
Thus there is still a need for organic semiconducting (OSC) materials that are easy to synthesize, especially by methods suitable for mass production, show good structural organization and film-forming properties, exhibit good electronic properties, especially a high charge carrier mobility, good processibility, especially a high solubility in organic solvents, and high stability in air. Especially for use in OPV cells, there is a need for OSC materials having a low bandgap, which enable improved light harvesting by the photoactive layer and can lead to higher cell efficiencies, compared to the polymers from prior art.
It was an aim of the present invention to provide new oligomers and polymers for use as organic semiconducting materials that do not have the drawbacks of prior art materials as described above, are easy to synthesize, especially by methods suitable for mass production, and do especially show good processibility, high stability, good solubility in organic solvents, high charge carrier mobility, and a low bandgap. Another aim of the invention was to extend the pool of OSC materials available to the expert. Other aims of the present invention are immediately evident to the expert from the following detailed description.
The inventors of the present invention have found that one or more of the above aims can be achieved by providing oligomers and conjugated polymers, containing dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene or other heterocyclic derivatives thereof, which are either tetrasubstituted or dialkylidene-substituted at the cyclopentane rings:
wherein R1 and R2 are e.g. alkyl or fluoroalkyl groups.
Strategically fusing additional aromatic rings along the long axis of the indacenodithiophene core unit creates numerous benefits in developing novel high performance OSC materials. Firstly, fusing additional aromatic rings increases the overall planarity and reduce the number of the potential twists of the conjugated molecular backbone. Elongation of a π-structure or monomer increases the extent of conjugation which facilitates charge transport along the polymer backbone. Secondly, increasing the proportion of sulphur atoms in the molecular backbone through fusing more thiophene rings promotes more intermolecular short contacts, which benefits charge hopping between molecules. Thirdly, the addition of fused-rings means increased proportion of ladder structure in the OSC polymer main chain, which improves the planarity of the molecular backbone. Additionally but not lastly, fusing aromatic rings can more efficiently modify the HOMO and LUMO energy levels and bandgaps of the target monomer structures compared with periphery substitutions.
Moreover, the dialkylidene-substituted dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophenes of the present invention are solubilised using alkylidene groups, of which the carbon atoms connecting to the ring systems are sp2-hybridized instead of sp3-hybridized. The sp2-carbons permit the solubilising alkyl chains to adopt a coplanar conformation relative to the core/polymer backbone, thus further facilitating cofacial aggregation in the solid state. This kind of coplanar orientation of the alkyls has been demonstrated by the crystal structures of compounds as disclosed in the examples of the present invention.
By the incorporation of the electron-donating dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene unitand an electron-accepting unit into a co-polymer i.e. a “donor-acceptor” polymer, a reduction of the bandgap can be achieved, which enables improved light harvesting properties in bulk heterojunction (BHJ) photovoltaic devices. Also, by varying the substituents at the cyclopentane rings, the solubility and electronic properties of the polymers can be further optimised.
JP 2010-280623 A1 discloses compounds of the following formula
wherein R1-R6 are C1-C30 alkyl. However, these compounds represent a significantly different attempt to solublise the dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene core structure, by placing alkyl groups R1-R6 on the terminal thiophene and the central benzene rings.
GB 2472413 A and WO 2012/017184 A1 describe small molecule materials with a general formula as follows
where Ar1 to Ar6 are independently fused heterocycles and T1 and T2 are terminal groups comprising both solublising chains and reactive functionalities.
However, there is no prior art disclosing oligomeric or polymeric materials containing dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene as claimed in the present invention.